The telecommunications industry is in the process of developing a new generation of flexible and affordable communications that includes high-speed access while also supporting broadband services. Many features of the third generation mobile telecommunications system have already been established, but many other features have yet to be perfected.
One of the systems within the third generation of mobile communications is the Universal Mobile Telecommunications System (UMTS) delivers voice, data, multimedia, and wideband information to stationary as well as mobile customers. UMTS is designed to accommodate increased system capacity and data capability. Efficient use of the electromagnetic spectrum is vital in UMTS. It is known that spectrum efficiency can be attained using frequency division duplex (FDD) or using time division duplex (TDD) schemes. Space division duplex (SDD) is a third duplex transmission method used for wireless telecommunications.
As can be seen in FIG. 1, the UMTS architecture consists of user equipment 102 (UE), the UMTS Terrestrial Radio Access Network 104 (UTRAN), and the Core Network 126 (CN). The air interface between the UTRAN and the UE is called Uu, and the interface between the UTRAN and the Core Network is called Iu.
The UTRAN consists of a set of Radio Network Subsystems 128 (RNS), each of which has geographic coverage of a number of cells 110 (C), as can be seen in FIG. 1. The interface between the subsystems is called Iur.
Each Radio Network Subsystem 128 (RNS) includes a Radio Network Controller 112 (RNC) and at least one Node B 114, each Node B having geographic coverage of at least one cell 110. As can be seen from FIG. 1, the interface between an RNC 112 and a Node B 114 is called Iub, and the Iub is hard-wired rather than being an air interface. For any Node B 114 there is only one RNC 112. A Node B 114 is responsible for radio transmission and reception to and from the UE 102 (Node B antennas can typically be seen atop towers or preferably at less visible locations). The RNC 112 has overall control of the logical resources of each Node B 114 within the RNS 128, and the RNC 112 is also responsible for handover decisions which entail switching a call from one cell to another or between radio channels in the same cell.
The FDD method uses separate frequency bands for uplink and downlink transmissions over the Uu interface (i.e. over the air interface between UTRAN 104 and the User Equipment 102). In contrast, the TDD method allocates different time slots (compared to different frequencies) for these uplink and downlink communications. Generally, TDD is very flexible regarding the allocation of time slots, and therefore is very well-suited to applications that are asymmetric with respect to uplink and downlink data volume (e.g. web browsing entails a much higher downlink than uplink data volume).
The TDD method uses the same frequency band but alternates the transmission direction in time. Further details can be found in the book WCDMA for UMTS; Radio Access for third Generation Mobile Communications, Third Edition, c. 2004, Edited by Harri Holma and Antti Toskala, and Chapter 13 titled “UTRA TDD Modes” is incorporated by reference herein. The term downlink or forward link refers to transmission from the base station (fixed network side) to the mobile terminal (user equipment), and the term uplink or reverse link refers to transmission from the mobile terminal to the base station. Since TDD uses the same frequency band but alternates the transmission direction in time, the frame structure of TDD needs to be carefully and smartly designed.
LTE, or Long Term Evolution (also known as 3.9G), refers to research and development involving the Third Generation Partnership Project (3GPP) aimed at identifying technologies and capabilities that can improve systems such as the UMTS. 3GPP is working on a standardization of LTE wherein both FDD and TDD duplex mode will be considered equally important.
3GPP TR 25.814, Physical Layer Aspects for Evolved Universal Terrestrial Access (UTRA) (Release 7), Version 7.0.0 (2006-6), is hereby incorporated by reference in its entirety. TR 25.814 defines two frame-structure options for LTE TDD; one of the two options is a frame structure compatible with a low chip rate (LCR) TDD, in order to accommodate coexistence with LCR-TDD. The LCR-TDD-compatible frame structure includes both data timeslots (TS) length TS0-TS6, and special timeslots position: downlink pilot timeslot (DwPTS), guard period (GP1), and uplink pilot timeslot (UpPTS).
Some characteristics specific to the TDD system are as follows. In either downlink or uplink, the transmission is discontinuous, switching between transmission directions requires time. Thus a Guard Period (GP) is needed in order to counter the propagation delay of the inter-site-distance (ISD) so as to avoid base station to base station interference. Timing advance can be used to avoid mobile terminal to mobile terminal interference. There is a wide consensus that the guard period (GP) should be variable and flexible to satisfy different inter-site distance (ISD). Another characteristic specific to the TDD system is that a downlink and uplink synchronization channel should be previously known to a mobile terminal, and provide high accuracy performance. A further characteristic specific to the TDD system is that the channel reciprocity in TDD should be utilized, with appropriate frame structure design. Design of the TDD frame structure should at least take these issues into account, in order to achieve high and robust performance.
According to TR 25.814, chapter 6.2.1.1.1, an alternative frame structure is listed for LTE TDD in order to co-exist with LCR-TDD. Unfortunately, the alternative frame structure described in chapter 6.2.1.1.1 does not adequately take into account the issues described above, and therefore does not achieve high and robust performance. There are several reasons for this. First, the maximum inter-site distance (ISD) is upper-bounded by a fixed value of GP1 (75 μs). Second, the coverage, performance and even functionality of a random access channel (RACH) could become degraded due to the short length for the RACH preamble, which is only 125 μs for TDD RACH compared with 1 ms for FDD RACH. Third, it is difficult and inefficient to utilize the non-synchronization-channel (i.e. non-SCH) and non-RACH subcarriers in DwPTS and UpPTS, respectively, or those subcarriers are in practice not even usable, so that the time domain occupancy efficiency is decreased for LTE TDD.
Thus, the alternative frame structure described in TR 25.814, chapter 6.2.1.1.1 has advantages and disadvantages. Among the disadvantages are the following four items.
First, the downlink synchronization channel (SCH) is always transmitted in the DwPTS of its central 72 subcarriers, according to R1-062786 CATT, “SCH Structure and Cell Search Method for E-UTRA TDD system”, Seoul, Korea, Oct. 9-13, 2006 which is hereby incorporated by reference in its entirety. Also incorporated by reference herein is R1-062785 CATT, Huawei, ZTE, RITT, “Consideration on the non-synchronized random access procedure for EUTRA TDD”, Seoul, Korea, Oct. 9-13, 2006. This means that, when operating at a higher channel bandwidth that corresponds to 72 subcarriers, the remaining subcarriers cannot be utilized for data transmission in any meaningful manner.
A second disadvantage is that the GP1 is fixed to 75 μs. Third, the uplink synchronization channel (RACH) is transmitted in UpPTS of its one or multiple 72 (or 12 depending on the final conclusion in 3 GPP) subcarriers. And, fourth, the timeslot interval (TI) at the end of TS0 is useless, because TS0 and DwPTS are always for downlink transmission.
Among the advantages of the alternative frame structure described in TR 25.814, chapter 6.2.1.1.1 are the following. First, the relative position between SCH and GP1 is fixed, and thus no extra signaling is needed to inform mobile terminals where the downlink-to-uplink switching point is. Second, the RACH is right after SCH and GP1, and the time distance between RACH and SCH is fixed and is the shortest possible, so that the channel reciprocity can be most efficiently utilized for initial non-synchronized RACH based on the latest channel information from SCH. Third, the exact timing enables co-existence with LCR-TDD.